Benedict XVI woke with his hair matted with blood after having a night-time fall during an official visit to Mexico last year, in an accident that contributed to his decision to step down as Pope.

The incident left the 85-year-old pontiff shaken, alarmed his doctors and precipitated his surprise decision to bring his eight-year old papacy to a close, Vatican sources said.

It happened when Benedict got up in the middle of the night to go to the bathroom while staying in the Mexican city of Leon. Finding himself in unfamiliar surroundings, he could not find a light switch and tripped, hitting his head against a wash basin.

When he woke in the morning his head and pillow were covered in blood.

The injury was not serious but caused grave alarm among his retinue.

It did not come to public notice because the cut on his head was covered by the mitre or skull cap that he wore for ceremonies during the rest of the trip.

"On the morning of March 25, the last day in Leon, where we were staying in a religious residence, Benedict XVI woke up and his hair was matted with blood," a priest who was on the trip told La Stampa newspaper.

"His aides asked him what had happened. The Pope said he had banged against the sink a few hours before. He woke up to go to the bathroom, but as often happens when you wake up in the middle of the night in an unfamiliar place, he couldn't find the light switch so he was in pitch darkness," the priest, who spoke anonymously, said.

"His pillow was covered in blood and there were a few drops on the floor," he added.

The accident, although not serious, had worried the Pope's doctor, Patrizio Polisca, who reportedly told him: "You see, Holy Father, why I disapprove of these trips?"

Father Federico Lombardi, the Vatican spokesman, confirmed on Thursday that the accident happened but denied that it had any bearing on the Pope's decision to resign – the first pontiff to willingly step aside since Celestine V in 1294.

It was "not relevant" to the historic resignation, he said.

As he prepares to formally resign at 8pm local time on Feb 28, the Pope said he would keep such a low profile during his retirement that he would be "hidden to the world".

"Even if I am withdrawing into prayer, I will always be close to all of you and I am sure that you will be close to me, even if I remain hidden to the world," he said in unscripted remarks to priests from the diocese of Rome.

The Vatican insists that there will be a clear break from the two papacies and that even though Benedict will live in a monastery within the walls of the city state, he will not interfere in the affairs of his successor.

But those assurances were called sharply into question on Thursday when it emerged that Georg Ganswein, Benedict's long-time confidant and fellow Bavarian, will remain his private secretary while also overseeing the new Pope's household. "He will remain prefect of the papal household and will also be secretary to Benedict," Federico Lombardi, the Vatican spokesman, said at a press briefing.

Not only will Monsignor Ganswein, 56, remain Benedict's secretary, he will also live with him in the monastery, tucked away in a corner of the Vatican gardens.

Msgr Ganswein's dual role will blur the lines between the old and new papacies and create even more confusion in a historically unprecedented situation which the Vatican is struggling to navigate.

The Pope's shock decision to resign was also motivated by bitter infighting between rival cardinals and divisions within the Curia, the Vatican bureaucracy.

Cardinal Julian Herranz, a member of the investigative commissionthe Vatileaks scandal – the theft and leaking of confidential documents by his butler, Paolo Gabriele - hinted its report had contributed to Benedict's resignation decision.

"Certainly there has been discussion of this being behind the Pope's resignation," he said.